The present invention relates in general to the construction of a stretched membrane mirror and to the method of manufacturing such a mirror. More particularly the present invention relates to the use of interconnected air actuated cylinders in order to uniformly distribute the tension across the surface of the membrane. While the primary application of the present invention is as a parabolic reflector for focusing solar energy on a receiver to convert the solar energy to electricity, the teachings of the present invention are not limited to this application.
In the context of the present invention, a stretched membrane mirror consists of a reflective film attached to both sides of a frame in a sealed manner which can then be deformed by the application of a vacuum in order to obtain a parabolic reflective surface. It has been shown that the amount and even distribution of tension in the film in the "flat state" is critical in the forming of the parabolic surface.
Earlier methods of manufacture have consisted of mounting the film in a fixture which has then been placed on the frame. Weights were then placed on the fixture which would cause the film to stretch over the frame. This fabrication process would produce inconsistent and uneven tension in the film besides distorting the frame under the force of the weights. There also appeared to be a practical limit to the tension that could be obtained in this way. The higher tensions required for an accurate parabolic surface could not be obtained with this process. Because of the nature of this method, it was typical for only one side or face of the mirror frame to be fabricated at a time. This method caused the mirror frame to be distorted or "potato chipped" when the fixturing device was removed, due to unequal forces on opposite surfaces of the frame.
After the film was stretched over the frame it would be sonic welded to a plastic insert in the frame to complete the process of film attachment. The process of sonic welding would sometimes damage the reflective surface causing a loss of the natural U.V. protection which is a critical factor for any outdoor use. As a consequence of this process, a tape would have to be applied to the edge of the film for U.V. protection. The tape protected the film not only from damage done to the film by the sonic welding processes, but also the edge of the film which is unmetallized. The process of sonic welding would also induce damage to the film structure caused by crystallization of the film polymer as a result of the elevated temperatures experienced during sonic welding.
In order to resolve these problems in manufacturing, a new technique according to the present invention has been developed. The present invention consists of a method to obtain high and consistent tension in the film and also provides for film edge protection in an innovative way. The section or panel of film that is to be attached to the circular mirror frame is gripped along nearly the entire perimeter by a series of individual pneumatic clamping assemblies. These assemblies are attached to air-actuated cylinders which are anchored to an outer support frame. Once the film is gripped by the pneumatic clamping assemblies, air pressure is applied to the air-actuated cylinders which in turn begin to place tension on the film. Since the air cylinders are interconnected, either with all other cylinders or with selected groups or pairs of cylinders, tension is evenly distributed at each of the clamps for each of the groups or pairs of interconnected air cylinders. Once the correct air pressure for the desired tension is determined for each air-actuated cylinder, repetitive tensioning of successive mirrors is performed by merely opening a valve from the corresponding regulator associated with each cylinder where that corresponding regulator is preset to the correct air pressure.
The pretension film is then bonded to the mirror frame by a two-part adhesive system. The first adhesive is a structural type that is designed for the long term. This type of adhesive typically has a cure time of several hours. The second adhesive is typically a cyanoacrylate type which is designed to immediately bond the film to the mirror frame in order to permit the quick removal of the film tensioning device so that it may be used to stretch and mount more film. The cyanoacrylate adhesive holds the film firmly to the mirror frame throughout the curing process of the structural adhesive. Before the film tensioner is removed and subsequent forces are applied to the mirror frame, the fabrication process is repeated for the opposite side. After the film is attached to both sides of the mirror frame, the tensioning devices can then be removed. Film edge protection is achieved by tucking the trimmed edge of the film into a groove in the mirror frame. This groove is then filled with a bead of silicone based sealant to complete the process.
In one arrangement of the film and mirror frame, the structural adhesive may be applied after the quick-set adhesive. This option is available when the structural adhesive is located on the mirror frame radially outwardly of the location of the quick-set (cyanoacrylate) adhesive. In practice the edge of the film which is not yet bonded to the mirror frame is lifted and an amount of structural adhesive is injected beneath the lifted edge of the film. If the film edge is to be located within a receiving groove or channel in the mirror frame, the next step is to locate the film edge in the groove. After the structural adhesive is cured, a final silicone sealant is applied to fill the groove.
Since a significant aspect of the present invention is the manufacturing method, it may of interest to consider other manufacturing methods associated with the forming and shaping of flexible materials. Consider for example the following patent references:
______________________________________ 3,429,006 Mattimoe et al. Feb. 25, 1969 3,878,010 Zammit Apr. 15, 1975 4,029,532 Warhol et al. June 14, 1977 4,490,205 Warhol Dec. 25, 1984 4,682,865 Rogers et al. Jul. 28, 1987 ______________________________________
Mattimoe discloses an apparatus for shaping a thermoplastic sheet for use in a curved laminated glass unit. Disclosed specifically is an automobile front windshield and the apparatus for shaping this member is a rectangular frame with moveable linkages and clamping means to hold onto the edges of the thermoplastic sheet.
Zammit discloses an apparatus and method for adhesively bonding a roof covering to an automobile vehicle roof. This apparatus also includes a fixturing frame involving a number of clamping devices. The apparatus also utilizes a female mold to which a vacuum is supplied in order to hold the roof covering in the configuration of the roof as the covering is moved over the roof during the bonding operation.
The '532 Warhol patent discloses a method of reshaping a flexible precut piece of vinyl or other flexible plastic and the processing of such a piece to form a stiffened preshaped assembly.
The '205 Warhol patent discloses an apparatus and method for shaping flexible material on a form. The flexible material is treated with a thermosetting composition and then applied to an automobile roof top form. Pneumatic tensioning means are attached to the edges of the material at a plurality of spaced-apart points to tension it over the form to a predetermined orientation and shape while maintaining the forces on the material substantially in equilibrium. The material is heated in order to set the thermosetting composition. Clamping means lightly engage the material to hold it in the predetermined shape untilthe setting temperature is reached and then firmly engage it to form marginal flanges thereon.
Rogers discloses a concave mirror apparatus and method of construction. This device includes a mirror having first, second, and third elongated strip portions in substantially parallel relationship to each other and spaced from each other with the third elongated strip portion located between the first and second elongated strip portions. The mirror is mounted on a mounting apparatus comprising first and second members for supporting the mirror along the first and second elongated strip portions, respectively.
Neither the fabrication method acknowledged by the inventors as already existing nor any of the fabrication methods disclosed in the above-listed patents involve the formation of a stretched membrane mirror by an arrangement of air-actuated cylinder groups which are interconnected and independently controlled by an air pressure regulator so that the tension across the entire surface of the reflective film will be the same. None of the described fabrication methods which are known to exist involve an application of the reflective film on both sides of the mirror frame so that a vacuum can be pulled in order to create a desired parabolic shape.
Further, the present invention recognizes that the film which is presently used is formed by an extrusion process and biaxial orientation and will stretch to different degrees under the same tension depending upon the anisotropy in film properties. There may be differences in the required stretching force for a uniform tension throughout depending upon whether the direction of stretch or tensioning is parallel (i.e., length) to the direction of extrusion or transverse (i.e., width) to the direction of extrusion. One theory suggests more of an elliptical pattern as well as an effect depending on where the panel of film was taken from the larger extruded roll. The use of different regulator networks accommodate these particular film characteristics even when a disk or circular panel of film is cut from the roll of extruded film.